1. Field of the Invention
The present invention relates to the field of power plant technology. It pertains to a method for operating a (stationary) gas turbine, as well as a gas turbine for implementing the method.
2. Brief Description of the Related Art
A gas turbine with reheating (reheat gas turbine) is known (see, for example, the U.S. Pat. No. 5,577,378 or “State of the art gas turbines—a brief update,” ABB Review February 1997, FIG. 15, turbine type GT26), which combines flexible operation with very low flue gas emission levels.
The machinery architecture of gas turbine type GT26 is unique and lends itself exceptionally well to the realization of a concept that is the subject matter of the present invention, because:
Even in the case of the compressor, there is a significant diversion of compressor air at intermediate compressor pressures,
The concept of sequential combustion renders increased stability of combustion possible, with decreased levels of excess oxygen, and
A secondary air system is present, which renders it possible to divert air from the compressor, to cool down, and to use the cooled air to cool the combustor and the turbine.
The principle of the known gas turbine with reheating is shown in FIG. 1. The gas turbine 11, which is a portion of the combined cycle power plant 10, includes two compressors, one connected behind the other, on a shaft 15 that is shared in common, namely a low pressure compressor 13 and a high pressure compressor 14, as well as two combustors, namely a high pressure combustor 18 and a reheat combustor 19, and pertinent turbines, namely a high pressure turbine 16 and a low pressure turbine 17. Shaft 15 drives a generator 12.
The way in which the unit works is the following: air is drawn in via an air inlet 20 by the low pressure compressor 13 and initially compressed to an intermediate level of pressure (ca. 20 bar). The high pressure compressor 14 then compresses the air further to a level of high pressure (ca. 32 bar). Cooling air is diverted, both at the intermediate level of pressure and at the level of high pressure, and cooled in pertinent OTC coolers (OTC=Once Through Cooler) 23 and 24 and conducted to the combustors 18 and 19 and turbines 16, 17 for cooling purposes via cooling lines 25 and 26. The remaining air from the high pressure compressor 14 is led to the high pressure combustor 18 and heated there by the combustion of a fuel brought in via the fuel feedline 21. The resultant flue gas is then expanded to an intermediate pressure level in the downstream high pressure turbine 16, as it performs work. After the expansion, the flue gas is heated again in the reheat combustor 19 by the combustion of a fuel led in via the fuel inlet 22, before it is expanded in the downstream low pressure turbine 17, performing additional work in the process.
The cooling air, which flows through the cooling lines 25, 26, is sprayed in at suitable points of the combustors 18, 19 and turbines 16, 17 to limit the material temperatures to a reasonable degree. The flue gas coming from the low pressure turbine 17 is sent through a heat recovery steam generator (HRSG) 27 to generate steam, which flows within a water-steam circuit through a steam turbine 29, performing additional work there. After flowing through the heat recovery steam generator 27, the flue gas is finally given off to the outside through a flue gas line 28. The OTC coolers 23, 24 are a portion of the water-steam circuit; superheated steam is generated at their outlets.
Great operational flexibility is achieved as a result of both combustions in combustors 18 and 19, which are independent of one another and follow one another sequentially; the combustor temperatures can be adjusted in such a manner that the maximal degree of efficiency is achieved within the existing limits. The low flue gas levels of the sequential combustion system are assured by the inherently low emission levels that can be achieved in conjunction with the reheating (under certain conditions, the second combustion even leads to a consumption of NOx).
On the other hand, combined cycle power plants with single-stage combustion in the gas turbines are known (see, for example, the U.S. Pat. Nos. 4,785,622 or 6,513,317), in which a coal gasifier is integrated, in order to provide the fuel that is necessary for the gas turbine in the form of syngas, which is recovered from coal. Such combined cycle power plants are characterized as IGCC plants (IGCC=Integrated Gasification Combined Cycle).
Now, the present invention proceeds from the knowledge that as a result of the use of gas turbines with reheating in an IGCC unit, the advantages of this type of gas turbine can be made usable for the unit in a particular manner.